1. Field of the Invention
The present invention relates to a screw rotor and a method of manufacturing the same, and more particularly to a screw rotor for use in a hydraulic machine which has a pair of rotors engageable with each other to compress fluid, pump fluid or expand fluid, and a method of manufacturing such a screw rotor.
2. Description of the Prior Art
There has been known a screw type of hydraulic machine in which a pair of screw rotors comprising a female rotor and a male rotor are engaged with each other and rotated in a casing, whereby fluid confined in cavities defined between outer surfaces of the screw rotors and an inner surface of the casing is transferred in an axial direction from one end of the screw rotors to the other end thereof, thus compressing fluid or pumping fluid.
In a screw rotor for use in a supercharger of an automobile or a compressor of an aircraft, there has been a strong requirement for reducing weight and moment of inertia thereof. To meet this requirement, there has been proposed a hollow screw rotor in, for example, Laid-Open utility model publication No. 63-198401. The hollow screw rotor disclosed in Laid-Open utility model publication No. 63-198401 is formed by a drawing process, an extruding process or a investment casting.
However, the screw rotor manufactured by the drawing process or the extruding process has the following various drawbacks:
(1) It is impossible to manufacture a screw rotor having a large helix angle.
(2) A screw rotor having a uniform helix angle and which is accurate cannot be formed.
(3) A screw rotor having a uniform thickness cannot be manufactured, which results in a deterioration of a dynamic balance.
(4) A screw rotor having small rounded corners or sharp corners on outer and inner surfaces thereof cannot be configured.
(5) A screw rotor having a thin thickness cannot be manufactured, thus failing to reduce weight and moment of inertia of the screw rotor.
The material to be used for a screw rotor is restricted because malleability is indispensable for the material.
On the other hand, the screw rotor manufactured by the investment casting is also problematic in that it is expensive in manufacturing cost and is not suitable for mass production because of its difficulty of removing sand from a product.